Nature has an order to it. The variations we find in our nature that conform to that order thrive, evolve, and continue on. Variations that go against that natural order do not. So even in nature we see a rudimentary good and a bad. We see an ideal and we see things that contradict that ideal (due to our fallen nature). And simply because something occurs in nature, doesn’t mean it is truly ordered to our human nature (as God designed) or necessarily good.
Here’s an analogy:
Dolphins, by their very nature, belong in the ocean – their biology is made to live that way. The fact that we sometimes “naturally” find a beached dolphin does not mean now that it is “natural” for a dolphin to be on the beach. Being out of water goes against the very nature of what it means to be a dolphin. And as much as the beached dolphin might like to rationalize that it is good to be on the beach, it can not make it so. And even if the beached dolphin decides to change the definition of the word “ocean” and have it include the sand as well as the water, it still can not change the reality of the natural order. All they will have done is caused the word “ocean” to lose its entire original meaning. And the beached dolphin is absolutely no better off than before. In fact, it’s in worse shape because it’s convinced itself that it’s okay to stay on the beach.
And that’s what we’re dealing with from those who are trying to redefine marriage. They find it inconvenient (and understandably so) that marriage is naturally ordered for the sea. And since they find this fact of life a very difficult boundary to accept, many would rather just redefine the boundaries instead.
But regardless of the nominal boundaries that any civil government wants to define, the reality of Marriage remains unchanged.
And yet, still, even though this basic reality is fully intelligible by the light of natural reason, the full beauty of the natural order can only be seen through the lens of its greater purpose. The Theology of the Body and the Church provide this most profound insight. And such truths, along with those of the natural order, help us to fully realize the significance of the sacrament of Marriage and how it fits into God’s love and plan for us.
For reference, here is what the Catechism says about homosexuality:
Homosexuality refers to relations between men or between women who experience an exclusive or predominant sexual attraction toward persons of the same sex. It has taken a great variety of forms through the centuries and in different cultures. Its psychological genesis remains largely unexplained. Basing itself on Sacred Scripture, which presents homosexual acts as acts of grave depravity, tradition has always declared that “homosexual acts are intrinsically disordered.” They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity. Under no circumstances can they be approved.
The number of men and women who have deep-seated homosexual tendencies is not negligible. This inclination, which is objectively disordered, constitutes for most of them a trial. They must be accepted with respect, compassion, and sensitivity. Every sign of unjust discrimination in their regard should be avoided. These persons are called to fulfill God’s will in their lives and, if they are Christians, to unite to the sacrifice of the Lord’s Cross the difficulties they may encounter from their condition.
Homosexual persons are called to chastity. By the virtues of self-mastery that teach them inner freedom, at times by the support of disinterested friendship, by prayer and sacramental grace, they can and should gradually and resolutely approach Christian perfection.